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Andamooka Opal


#1
I am wondering about the on Andamooka opal. I am
slightly familiar with the material, and did not think that all of
it was treated.  Can anyone verify this one way or the
other? 

Andamooka is a famous Australian opal-mining area that is currently
not producing much. Its orange-green opals are famous and in a class
by themselves, and in its heyday it produced more light and crystal
opals than any other location in Australia. I think the material
referenced by Mr. Barzilay is Andamooka matrix opal, a sandstone
suffused with precious opal. It’s also called “cooking opal” because
the porous light-colored material after cutting is heated in a bath
of sugar solution, then plunged into acid. The acid carbonizes the
impregnated sugar, turning it black, which highlights the opal
colors. I’ve cut and treated a lot of this material. It can be quite
beautiful and has its own range of values based on the amount, type
and intensity of the colors and disclosure of treatment.
Unfortunately it is sometimes misrepresented as “black opal” (not the
case in this instance however) and offered at unrealistic high
prices. It cannot be re-cut without re-treating because the black
layer is only a fraction of an inch deep. Similar matrix occurs at
several other locations and there are several treatment methods

Rick Martin
MARTIN DESIGNS


#2
    Andamooka matrix opal, a sandstone suffused with precious opal.
It's also called "cooking opal" because the porous light-colored
material after cutting is heated in a bath of sugar solution, then
plunged into acid. The acid carbonizes the impregnated sugar,
turning it black, 

G’day; I had to poke my old nose in to this one, though it isn’t
really incorrect. (the not my nose) First of all the
sugar solution should be fairly strong (around a heaped tablespoon in
a cup of hot water.) Next, it would be best if the matrix material
could be either placed under vacuum whilst in the sugar solution, or
put under high pressure; either way the solution needs time to be
driven into the material. It must be removed from the sugar, dried,
then placed in a dish, and concentrated sulphuric acid poured over it
and left a while. It will get very hot, with steam and smoke and
smell - well, like burning sugar! It is only very strong sulphuric
acid that will remove all the water from the sugar to leave black
carbon.

A molecule of sugar is C12H22011 The conc acid removes the H220ll
bit - which is simply ll molecules of H20 - water - check it - and
therefore leaves the 12 atoms of carbon behind.

The material would need to be neutralised with hot washing soda or
baking soda solution then thoroughly washed and dried. – Cheers for
now,

John Burgess; @John_Burgess2 of Mapua Nelson NZ


#3

Hi all. Thought I would share this info. from the opal gem dealers
I buy from. I wanted a clerification from them and thought I would
send it on. Great people to do business with. Opalcentric- email
tina_heaney@excite.com in Bondville,Vt. I believe they will be in
Tuscon. Louise